These studies aim to 1) elucidate structural aspects of prothrombin affecting its ability to activate to thrombin, 2) determine some of the mechanisms controlling the in vivo metabolism of human prothrombin, Factor IX and Factor X, 3) determine the fate of thrombin, Factor IXa and Factor Xa generated during clotting. In vivo studies will make use of radiolabelled purified normal human prothrombin, Factor IX and Factor X, genetically abnormal prothrombins (Cardeza, San Juan I and II, Padua, Molise), and a genetically abnormal Factor X. In vitro studies will make use of radiolabelled thrombin, Factor IXa and Factor Xa, in addition. Abnormal fragments found in the serum of the genetic dysprothrombinemias, or produced in purified activation mixtures, will be sequenced. We shall continue studies of factors underlying the formation of antibodies to Factor VIII in hemophiliacs and in some normal individuals. The character of the antibody population will be examined more closely, particularly as a function of time after antigenic (Factor VIII) challenge. The nature of the heterogeneity found in isoelectric focusing will be investigated in more detail. Investigation of host factors will continue: HLA-phenotyping of the hemophilic population will be expanded to include HLA-C and HLA-D. In vitro lymphocyte immunization techniques will be used in an effort to predict subsequent in vivo antibody formation, and to examine the possibility that multiply tranfused patients who do not form antibodies may be tolerant to Factor VIII.